The subject matter relates to a dual-crankshaft, opposed-piston engine with improvements for variable port timing and gear train resonance reduction. More particularly, the subject matter relates to an opposed-piston engine with two crankshafts coupled by a gear train, in which the crankshafts are coupled together by a timing control mechanism that acts between the crankshafts to vary the timing of port operations in the engine. In other aspects, the subject matter relates to an opposed-piston engine with two crankshafts coupled by a gear train, in which vibration of the gear train occurring at various engine speeds is reduced.
In an opposed-piston engine, a pair of pistons is disposed for opposed sliding motion in the bore of at least one ported cylinder. Each cylinder has exhaust and intake ports, and the cylinders are juxtaposed and oriented with exhaust and intake ports mutually aligned. Of two crankshafts, one each is rotatably mounted at respective exhaust ends and intake ends of the cylinders, and each piston is coupled to drive a respective one of the two crankshafts. The reciprocal movement of each piston in the cylinder controls the operation of a respective one of the two ports formed in the cylinder's sidewall. Each port is located at a fixed position where it is opened and closed by a respective piston at predetermined points during each cycle of engine operation.
It is desirable to be able to vary the timing of port openings and closings during engine operation in order to dynamically adapt the time that a port remains open to changing speeds and loads that occur during engine operation. The objective is to maximize the amount of air trapped in the cylinder during the compression stroke during various phases of engine operation.
In a dual-crankshaft, opposed-piston engine architecture, the trapped compression ratio (trapped CR) can be varied by adjusting the phase offset between the exhaust and intake crankshafts. Increasing the exhaust crank lead from a nominal value results in decreasing the trapped compression ratio along with a corresponding increase in the exhaust blowdown time-area, that is, the time-integrated area that the exhaust port is open before the intake port opens. Conversely, decreasing the exhaust crank lead results in increasing the trapped compression ratio along with a corresponding decrease in the exhaust blowdown time-area.
Concurrently decreasing the trapped compression ratio and increasing the exhaust blowdown time-area is advantageous for standard engine operation at high engine speeds and high engine loads. At these conditions, lower trapped compression ratios are typically desired because of NOx emission considerations (lower CR typically leads to lower NOx emission), while larger blowdown time-areas are required because of the decreased wall-clock time available to blow down the cylinder contents into the exhaust manifold prior to the intake ports opening.
Similarly, the concurrently increasing trapped compression ratio and decreasing exhaust blowdown time-area is advantageous at lower speeds and lower loads, where higher compression ratios are advantageous for cold-start and engine efficiency considerations and where less exhaust blowdown time-area is required.